A flush joint tubular connection has an outer diameter substantially the same as the tubing joints which the connection joins.
Standard API non-upset tubing connections comprise couplings having outer diameters considerably larger than the pipe outer diameter but still only enjoy approximately 42% efficiency as above. API does list a "turned down" collar outer diameter to increase clearance between strings; however, the "turned down" diameter still exceeds substantially, the piper outer diameter.
No prior art discloses a flush joint tubular connection having tapered threads, that when properly assembled, effects optimum stresses within the small end of the external thread and within the large end of the internal thread so as to provide a connection of maximum efficiency. Conventional pipe connections have threads with like tapers and result in a constant diametrical interference along the taper between the external and internal threads, thereby causing excessive stresses or requiring increased wall thickness at the end of the pipe. Excessive stresses reduce the joint strength and an increased wall thickness rules out a flush joint connection.
Historically, the most popular flush joint tubular connection is made by the Hydril.RTM. Co., and covered by numerous patents and comprises straight threads and a tapered mating seal between the two joints of tubing resulting in a premium joint of high cost and according to published data, enjoys only 42% axial strength with regard to the pipe wall. Since the straight threads cannot seal against fluid pressure, a seal against internal fluid pressure is provided on a pitch circle smaller than the pitch circle of the threads and a seal against external fluid pressure is provided on a pitch circle larger than the pitch circle of the threads. Upon tightening of the joint, the seal of least diameter contacts before the seal of greatest diameter, the seal of greatest diameter acting as a stop shoulder to limit tightening of the connection. However, since the non-upset wall of the pipe must be radially divided into at least three parts (i.e. inner seal, thread, and outer seal) the radial width of the stop shoulder must be substantially less than the wall thickness of the pipe. Also, the shoulder material can be no stronger than the pipe materail to increase torque capacity, because it is formed on the non-upset end of the pipe. Additionally, when pipe compound used to lubricate the connection is trapped between the threads and the outer seal, incomplete make-up may result because no provision is made to allow escape of the thread compound during make-up. When complete make-up does occur with thread compound trapped between the inner and outer seals, thermal expansion of the thread compound can cause excessive pressure there between to result in damage to or disengagement of the threads. With the high lubricity of the thread compound acting between the stop shoulders, counter torque between shoulders of limited dimension is lower which may allow undesirable overtightening of the connection. When the connection is overtightened, the stop shoulders fail in bearing and loosening and leakage can then occur and the neck section of the external thread may be greatly overstressed and thereby cause premature failure of the connection. It is therefore clear that a flush joint tubular connection having a wider acceptable range of make-up torque is needed for use within oilwells.
It is also clear that a flush joint connection having a high axial strength and a more certain ability to seal as provided by the instant invention is needed for use within oilwells and in other pipe assemblies wherein radial clearance is crucial.